Soft X-ray blast windows (or filters) transmit soft X-rays but hold back the blast from an explosive X-ray source. Unlike hard X-rays, such as those conventionally used in medical X-ray devices, soft X-rays do not transmit well through most materials and are instead absorbed by the material. Generally, soft x-ray blast windows are used for conducting soft X-ray tests in laboratory radiation simulators. The earliest soft X-ray windows were foils from either beryllium or strong plastics known to transmit at least some soft X-rays, such as KAPTON, MYLAR or Kimfol. However, due to its toxic nature, beryllium becomes a health hazard when it is vaporized by the radiation from an explosive X-ray source. Thus, beryllium is not suited as a soft X-ray blast window. Moreover, beryllium is expensive. Meanwhile, the plastics discussed above are low-cost, but not nearly as strong as beryllium and they absorb soft X-rays much more than desirable.
Lower atomic number materials absorb soft X-rays less, and therefore make better windows for soft X-rays. However, these nontraditional materials bring with them many problems. For example, the highest X-ray transmission is offered by solid deuterium, the lowest atomic number material, and X-ray filters have been developed from solid deuterium. While a solid deuterium window transmits even the softest X-rays of interest when testing surfaces for their response to intense radiation pulses, solid deuterium windows are stable only at cryogenic temperature below about 6K and under vacuum conditions, which are expensive and cumbersome.
Thus, a soft X-ray window was developed from pure lithium metal, a low atomic number material that is a solid at room temperature. While lithium will transmit soft X-rays, but not as well as deuterium does, the majority of soft X-rays useful for testing will transmit through lithium. Also, lithium is relatively stable making it a good material for blast windows. However, lithium is not a particularly strong material. To address the problem of strength, lithium windows are made more effective in transmitting soft X-rays by supporting them on a compliant grid of strong wires. The grid compensates for the low strength of the lithium metal itself in the same way that nylon wires reinforce strapping tape. However, the wires are made from higher atomic number material, such as stretched polyethylene, and thus absorb some of the soft X-rays, reducing overall soft X-ray transmission. In addition, the lithium material can be made stronger by cooling the material to 77 K. At such cryogenic temperatures, all metals become stronger, harder and more brittle, but lithium remains ductile down to at least 77K. Thus, a grid support for lithium at 77K can have wires that are farther apart than a grid support at room temperature, letting more X-rays through, without sacrificing strength of the window. However, cryogenic cooling requires additional equipment, expense and is sometimes considered too complicated for implementation by operators not accustomed to dealing with cryogenic systems. Beryllium, on the other hand, becomes too brittle at cryogenic temperatures to be effective.
It is therefore desirable to find alternatives that have effective X-ray transmission similar to lithium while having increased strength even without cryogenics. It would be an additional advantage if this same material were to show the additional increase in strength on cryogenic cooling and/or when placed on a grid as lithium does.